feat: add set as wallpaper functionality

Add ability to set current image as desktop wallpaper with keyboard
shortcut 'W' and icon button in Properties panel.

Supports COSMIC, GNOME, KDE, XFCE, and tiling window managers via
automatic detection and fallback mechanism.

Implementation uses wallpaper crate with custom COSMIC config file
integration and gsettings/feh fallbacks.

docs/usage.md

@@ -75,6 +75,12 @@ All transformations are lossless and show in real-time.
 | `i` | Toggle properties      | Show/hide the properties panel (metadata)|
 | `n` | Toggle navigation      | Show/hide the navigation sidebar         |
 
+### Actions
+
+| Key | Action                 | Description                              |
+|:----|:-----------------------|:-----------------------------------------|
+| `w` | Set as wallpaper       | Set the current image as desktop wallpaper|
+
 ## Mouse Controls
 
 ### Zoom
@@ -101,6 +107,26 @@ The header toolbar provides quick access to common operations:
 ### Right Side
 - **Properties toggle**: Show/hide the metadata panel
 
+## Properties Panel
+
+The properties panel (toggle with `i`) displays image metadata and provides quick actions:
+
+### Action Buttons
+Located at the top-right of the properties panel:
+
+- **Set as Wallpaper** (`w` key): Set the current image as your desktop wallpaper
+  - Works with COSMIC, GNOME, KDE, XFCE, and tiling window managers
+  - Automatically detects your desktop environment
+  - Falls back to alternative methods if the primary method fails
+  
+- **Open With** (planned): Open the image with another application
+
+- **Show in Folder** (planned): Open the containing folder in your file manager
+
+### Metadata Display
+- **File Information**: Name, format, dimensions, file size, color type
+- **Camera Information** (if available): Camera model, date taken, exposure settings, GPS location
+
 ## Footer Information
 
 The footer displays useful information: